It was 17 to buy airguns and different for pellets. Age was 14 for pellets . A person over the age of 13 (possibly 14) could possess and use on private land. Under 13 could not own or possess either but could use a gun whilst under the supervision of someone over the age of 21.
When this changed I do not know but I think this century. The firearms act 1968 has been ammended so many times I've lost track. It was the same with shotguns until 1968

Airguns were at odds with firearms as there was no age limit for a firearms certificate but one had to be over 17 to buy ammunition on certificate.

When originally set, the age limits took youth groups such as scouts and cadets into account